William Mallet

To examine TGN38 trafficking from the cell surface to the TGN, CHO cells were stably transfected with a chimeric transmembrane protein, TacTGN38. We used fluorescent and 125I-labeled anti-Tac IgG and Fab fragments to follow TacTGN38's postendocytic trafficking. At steady-state, anti-Tac was mainly in the TGN, but shortly after endocytosis it was predominantly in early endosomes. 11% of cellular TacTGN38 is on the plasma membrane. Kinetic analysis of trafficking of antibodies bound to TacTGN38 showed that after short endocytic pulses, 80% of internalized anti-Tac returned to the cell surface (t1/2 = 9 min), and the remainder trafficked to the TGN. When longer filling pulses and chases were used to load anti-Tac into the TGN, it returned to the cell surface with a t1/2 of 46 min. Quantitative confocal microscopy analysis also showed that fluorescent anti-Tac fills the TGN with a 46-min t1/2. Using the measured rate constants in a simple kinetic model, we predict that 82% of TacTGN38 is in the TGN, and 7% is in endosomes. TacTGN38 leaves the TGN slowly, which accounts for its steady-state distribution despite the inefficient targeting from the cell surface to the TGN.

Furin and TGN38 are menbrane proteins that cycle between the plasma membrane and the trans-Golgi network (TGN), each maintaining a predominant distribution in the TGN. We have used chimeric proteins with an extracellular Tac domain and the cytoplasmic domain of TGN38 or furin to study the trafficking of these proteins in endosomes. Previously, we demonstrated that the postendocytic trafficking of Tac-TGN38 to the TGN is via the endocytic recycling pathway (Ghosh, R.N.,W.G. Mallet,T.T. Soe,T.E.McGraw, and F.R. Maxfield.1998.J. Cell Biol.142:923-936). Here we show that internalized Tac-furin is delivered to the TGN through late endosomes, bypassing the endocytic recycling compartment. The transport of Tac-furin from late endosomes to the TGN appears to proceed via an efficient, single-pass mechanism. Delivery of Tac-furin but not Tac-TGN38 to the TGN is blocked by nocodazole, and the two pathways are also differentially affected by wortmannin. These studies demonstrate the existence of two independentpathways for endosomal transport of proteins to the TGN from the plasma membrane.

Although the distribution of the cation-independent mannose 6-phosphate receptor (CI-MPR) has been well studied, its intracellular itinerary and trafficking kinetics remain uncertain. In this report, we describe the endocytic trafficking and steady-state localization of a chimeric form of the CI-MPR containing the ecto-domain of the bovine CI-MPR and the murine transmembrane and cytoplasmic domains expressed in a CHO cell line. Detailed confocal microscopy analysis revealed that internalized chimeric CI-MPR overlaps almost completely with the endogenous CI-MPR but only partially with individual markers for the trans-Golgi network or other endosomal compartments. After endocytosis, the chimeric receptor first enters sorting endosomes, and it then accumulates in the endocytic recycling compartment. A large fraction of the receptors return to the plasma membrane, but some are delivered to the trans-Golgi network and/or late endosomes. Over the course of an hour, the endocytosed receptors achieve their steady-state distribution. Importantly, the receptor does not start to colocalize with late endosomal markers until after it has passed through the endocytic recycling compartment. In CHO cells, only a small fraction of the receptor is ever detected in endosomes bearing substrates destined for lysosomes (kinetically defined late endosomes). These data demonstrate that CI-MPR takes a complex route that involves multiple sorting steps in both early and late endosomes.